“Sales tax basis” unclear

Question regarding the value entered for “Sales tax basis"

When establishing System Costs, Indirect Capital Costs include sales tax, which appears to be calculated by applying the sales tax rate entered in the Financial Parameters x the Total Direct Captial Costs, but also modified by the value entered in the field “Sales tax basis” as a percentage.
The default value is 82, and it is unclear where this default value comes from or how it should be user modified.

I would expect the sales tax to be applied only to the equipment cost components of the Direct Capital Costs (i.e. Module, Inverter, and BOS equipment). I can manually calculate the percentage of Direct Capital Costs comprising equipment, but I’m surprised SAM doesn’t do this automatically or provide guidance on the input.
Am I missing something?

Hello,
No, you are not missing anything. Your description of the sales tax calculation is correct.
The default value of the sales tax basis is different for the different performance models. For the CSP models in SAM 2015.1.30, it is 80%, and is a rough estimate of the portion of the direct cost assumed to be materials. For more about the default costs, see the documents on the System Costs page of this website.
As with all of the inputs in SAM, the default values are rough estimates to help you get started using the software. You should review all of the values to ensure that they are appropriate for your analysis.
Best regards,

Paul.